Geochemistry of Some Hp-Metavolcanics from Western Alps Metaophiolites

Acta Mineralogica-Petrographica, Szeged. XXX, 55—65,1989 GEOCHEMISTRY OF SOME HP-METAVOLCANICS FROM WESTERN ALPS METAOPHIOLITES I. KUBOVICS and ABDHL AAL M. ABDEL-KARIM Department of Petrology and Geochemistry, Lóránd Eötvös University ABSTRACT Ophiolite metavolvanics from Monviso, Arc valley and Montgenévre in the Piedmont Zone of the Western Alps are geochemically investigated and compared with data obtained from oceanic crust. Belonging to the Zermatt-Saas and Combin (Monviso and Arc valley) units they are characterized by HP-facies metamorphism including eclogite-facies metabasalts and garnet-bearing glaucophanites that underwent into greenschist-facies metamorphism. Few metavolcanics (Montgenévre) are partly escaped from ocean-floor metamorphism, but they are overprinted by Alpine deformation. They show a large variation in the bulk rock geochemistry and roughly compare to MORB and a few of them show an IAT character. Their geochemical characters prevalently similar to that of oceanic ridge basalts and they show abyssal tholeiitic differentiation trend. KEYWORDS: HP-Facies metavolcanics, Geochemistry, metamorphic evolution, Peidmont Zone, Western Alps metaophiolite. INTRODUCTION * The HP-metamorphism had an effect on large bodies of the pre-Alpine continental and organic crust in the Western Alps. During the Cretaceous, most of the oceanic crust (now represented by the Piedmont ophiolite nappe) and some parts of both continental margins underwent blueschist to eclogitic conditions that are followed by the greenschist facies conditions (LOMBARDO, 1988). The geochemistry of the metavolcanics from the Western Alps have been sub- jected to various studies ; previous researches had carried out from Monviso in Cottain Alps (LOMBARDO et al., 1978, NISIO, 1985, NISIO and LARDEAUX, 1987, COMPAGNONI et al., 1988), Arc valley in the Zermatt-Saas zone (BOCQUET, 1974, DAL PIAZ et al., 1981; DEN TEX, 1987; LEARDI et al., 1986) and from Chenaillet in Montgenévre (MÉVEL, 1975, LEWIS and SMEWING, 1980, BERTRAND et al., 1981, 1982, 1987). FIELD RELATIONS Field study and sampling of the metavolcanics were carried out in the eastern, central and western parts of the Piedmont ophiolite nappe in the Western Alps, e.g. Monviso, Arc valley area and Montgenevre ophiolites (Fig. 1.). The Piedmont ophiolite is represented by thinned, sheared, multistage folded and metamorphosed rem- nants of a narrow oceanic crust and related upper mantle (DAL PIAZ et al., 1981). H-1088 Budapest, Muzeum krt. 4/a, Hungary. 55 Fig. 1. Tectonic sketch map of the internal Western Alps showing the location of the main ophiolite complexes. 1. Dora-Maire (DM) and Gran Paradiso (GP) continental units (European Paleomargin), 2. Vanoise, Ambin (AM) and Brianconnais (B), continental units (European Paleomargin) Mesozoic epicontinental covers, 4. Piedmont Zones (PZ): Schistes Lustres'nappe (Mesozoic, mainly oceanic material): a. undifferentiated metasediments with subordinate ophiolites; b. ophiolite complex with minor metasediments; c. metagabbro bodies. Location of samples: RA Refuged'A verole;PC=Pre clos la Clapera; CP-MC=Carrieres du Paradis-Mont Cenis; PB=Petit Belvedere; LL=Lago Lausetto; CF=Colleto Fiorenza; C=Chenaillet 56 Some of these metavolcanics are partly escaped from the strong Alpine deformation while others have no primary minerals of features. Belonging to the Combin and Zermatt-Saas Units they are in different structural and stratigraphic settings compared with other parts of the ophiolite sequences (peridotite, gabbros and metasediments). The overlying Combin unit displays pre-ophiolitic basal complex of Triassic to Liassic age with epicontinental affinity coverd by a thick volcanoclastic ophiolite bearing sequence (DAL PIAZ, 1974; DAL PIAZ et al, 1981; BEARTH, 1967; ETLER, 1971) which consists of metasediments and interbedded basaltic metavolcanics. This unit is also implied its relationship with other ophiolite sequences and characterized by an ocean-floor metamorphism which was overprinted by greenschist facies (HUN- ZIKER, 1974; DAL PIAZ et al. 1981). The underlying Zermatt-Saas unit consits of basal serpentinite, gabbro capped by retrograde eclogitic basalt and garnet-bearing metasediments (BEARTH, 1973; DAL PIAZ, 1974; DAL PIAZ et al., 1978, 1981). This unit has been deformed and affec- ted by HP-metamorphism and greenschist facies conditions and are considered as oceanic crust (BEARTH, 1967, DAL PIAZ, 1974, DAL PIAZ et al., 1978, 1981. e.t.c.). The Monviso metavolcanics show three successive Alpine metamorphic stages characterized by eclogitic, blueschist and greenschist facies. They include fine grained eclogitic metabasalts and banded metabasites (garnet-bearing glaucophanites and prasinites). From few cm-s up to several dm-s thick eclogitic metabasalts collected in Colletto Fiorenza, are crosscut by smaragditic metagabbros and younger albite 20 * ^ 15 o o tlaj. + + • ID- + • +V > T ¥ al- T Ax X * A + X 5 S' * io- vO m tf- 7,5- • V <3, 5- * + • + 2: 1- o 2,5- o AV + 70 VP V o + O*- 60 V •A •AA • * o *i • * + OCO? 50 * 40 0 05 0,6 0,7 0.8 0,9 0 0,6, Q7 OS 09 FeO+/ Fe0+' MgO ratio Fig. 2.: Major element oxide wt% versus FeO+ :eO++MgO ratio diagram for ophiolitic metavolcanics from Western Al i. (Symbols as in Table 1) 57 veins. The garnet-bearing glaucophanite in Petit Belvedere is usually heterogeneous and it shows banded layering. The prasinite (Lago Lausetto) represents a homogenous thin layer (mm-scale) which differs in colour and mineral composition. The Monviso metavolcanics appear to belong to the Zermatt-Saas unit. The Arc-valley metavolcanics of Combin unit consist of up to several hundered meters thick homogeneous prasinite which sometimes overlies on a thin ovardite level (the ovardite occurs as some thin interlations, too) and usually associate with garnet-bearing glaucophanite. They problably correspond to submarine flows, hyaloclastites and tuffites. The metavolcanic sequence of Montgenevre represents the best preserved ophiolite complex in the Western Alps (BERTRAND et al., 1987) and occurs as a separate tectonic unit. The pillow lavas are detached from the gabbros by a shear zone and sporadically contain serpentinite lenses. PETROGRAPHY The primary and secondary mineral assemblages, texture and metamorphic facies of the discussed metavolcanics are summarized in Table 1. CHEMISTRY Twelve selected metavolcanic samples collected from Monviso, Arc valley and Montgenevre in Western Alps were analysed. With the aim of the comparison we adapted six published analyses, as well. The location of samples are plotted in Fig. 1. and the chemical data appear in Table 2. a) Analytical Methods The silica was determined thermogravimetrically using the SAJO'S method <1955). The ferrous Fe was analysed by HOFFMANN'S method (301/87 OTH Patent). Ferric Fe was computed by the difference between FeO and total Fe, Al, total Fe, Mg, Ca, Na, K and Mn were measured by atomic absorption spectrophotometry. Other elements were determined by spectrophotometric method. H20, C02, and S02 were analysed by DTA method. The analyses were carried out by L. Hoffmann in the Department of Petrology and Geochemistry, Eötvös L. University, Budapest. b) Results The analysed HP-facies metavolcanics include 2 eclogitic metabasalts of Monviso (COMPAGNONI et al., 1988) and 3 garnet-bearing glaucophanites of Monviso and Arc valley (Table 2). The eclogitic metabasalts have large amounts of Si02 (50.61 */Al203/l 5.6 *) and Mno (0.48*) and small amounts of FeO+(8,47*) and Ti02 (1,4*). The garnet-bearing glaucophanite of Monviso differs from those of Arc valley in its much higher Ti02 (3.23*) and FeO+ (20.38*) and much lower A1203 (8.67*) and NaaO (3.65*) content, which may be ascribed to the abundance of Fe—Ti rich 58 TABLE 1 Main pétrographie features of representative rocks from Western Alps metavolcanics Main primary minerals Main secondary minerals Rock name Texture (magmatic (hydrothermal and meta- Metamorphic fades and late-stage morphic) primary) 1. MONVISO: Coletto Fiorenza Eclogitic porph, gran cpx omp, gar, rut, clinoz, glau, Eclogitic f. metabas. Mg-chl, leuc, ab, phen, actin, Fe-chl,tit. Petit Belvedere Gar.-bearing granone- glau, leuc, ep, chl, ab, Glaucophane 5= glaucophanite matobl, stilp, actin, gar. schist f. O poikil, sch. Greenschist poikil, actin, chl, ab, tit, clinoz, Greenschist f. nematobl. cc. a Lago Lausetto o. Prasinite poikil, ab, chl, ep, glau, actin, Greenschist f. x phen, rut-tit. 2. ARC VALLEY Gar.-bearing granone- glau, ep, chl, ab, gar, tit, Glaucophane matobl, schist f. Glaucophanite poikil, sch. phen, cc, qz, bio. Prasinite poikil, sch. ab, ep. actin, rieb, glau, Greenschist f. chl, rut-tit, cc, phen, bio. Ovardite poikil, porp, ab, Fe-chl, glau, actin, Greenschist f. gne. ep, cc, rut-tit, phen, qz±gar. 3. MONTGENÉVRE: Chenaillet Metabas. pill, inters, porp, cpx, pi ab, chl, preh, ep, cc±zeol, lava aphy, que, pump. vario, arb, ves. Metabas. pill, inters, aphy, cpx, pi, epi, preh, chl, ab, hem± low-grade met.-ocean breccia vario pump. floor met. Metadol. lava vario, su- Prehn.-pump. and flow and dyke boph, interg, ol, cpx, pi, ab, chl, leuc, cc±preh greenschist facies Microgabbro hypid cpx, pi. hb. chl, actin-trem, leuc, ap, Abbreviations: ol: olivine; cpx: clinopyroxene; pi: plagioclase; omp: omphacite; glau: glaucophane; actin: actionlite; trem: tremolite; rieb: riebeckite; hb: hornblende; ep: epidote; clinoz: clinozoisite; chl: chlorite; preh: prehnite; cc: calcite; ap: apatite; hem: hematite; leuc: leucoxene; rut: rutile,

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